The Research: Energy storage, maintenance and utilization is critical to cellular and muscle function. It has been thought that creatine kinase plays a crucial role in this process by providing an "energy reserve" function, however the compensatory mechanisms and molecular responses to deficiencies of the creatine kinase system are incompletely understood. Based on the traditional understanding of the creatine kinase system one would expect that animals lacking M creatine kinase would be severely impaired or unable to survive. Surprisingly, recent studies of cardiac and skeletal muscle of mice which express no M creatine kinase, demonstrate no noticeable abnormalities at rest, or under conditions of low ATP flux, suggesting that this is not flee case. When a heart from one of these mice is subjected to acute isoproterenol infusion (high ATP flux), there appears to be an "uncoupling" of ATP and creatine phosphate. The hypothesis of this proposal is that M creatine kinase activity is of crucial importance in maintaining myocardial energy metabolism homeostasis in high flux stress states. The applicant has developed an isolated perfused mouse heart preparation, to which he will apply a multidisciplinary approach integrating sophisticated biochemical techniques (31P NMR spectroscopy), physiologic and molecular techniques to study the following Specific Aims: l) Are concentrations of ATP and creatine phosphate uncoupled in hearts from M creatine kinase homozygous knockout mice under high flux but not under low flux stress? 2) Can the coupling of ATP to creatine phosphate concentrations in muscle cells subjected to low and high flux stress be altered by the over-expression of creatine kinase isoenzymes or of the creatine transporter? 3) Does the absence of functional M creatine kinase alter the expression of other genes as a compensatory mechanism in M creatine kinase homozygous knockout mouse hearts under basal conditions and under conditions of low and high flux stress?